Propene (C3H6), often also referred to as propylene, is one of the most important starting substances of the chemical industry. The demand for the base material propylene is increasing worldwide, wherein propylene just like ethylene mostly is produced from petroleum in a steam cracker in a ratio dependent on the process and the raw materials.
To obtain additional propylene, a number of processes exist, such as the PDH process which proceeds from propane as educt. However, since the largest part of propylene still is produced by steam cracking (about 70%), there is a tendency to convert the C4 to C8 olefins obtained in crackers or other petrochemical plants to additional propylene, in part also to ethylene.
On the one hand, this can be effected via the metathesis process, which is based on a synproportionation of ethylene and butylene. It is disadvantageous here that for this purpose the ethylene production must be increased and only C4 olefins can be converted.
Furthermore, an olefin conversion is possible, in which C4+ olefins are converted to propylene. Such cracking is effected by means of the Propylur or OCP process and above all is employed to utilize the C4+ olefins produced in a cracker plant—which are to be valued comparatively low—for the production of propylene. Due to the endothermicity of the reaction, the temperature in the reactor however decreases with increasing conversion and thus limits the achievable propylene yield.
Finally, the methanol-to-propylene process (also MTP® process) is recommendable, in which methanol/dimethyl ether or also other oxygenates are converted to propylene on a mostly zeolitic catalyst.
DE 10 2005 048 931 describes such MTP® process for the production of C2 to C4 olefins from an educt mixture containing steam and oxygenates, such as methanol and/or dimethyl ether, in which the educt mixture is converted in at least one reactor by a heterogeneously catalyzed reaction to obtain a reaction mixture comprising low-molecular olefins and gasoline hydrocarbons. In a first separating means, this reaction mixture then is separated into a mixture rich in C5− olefins, a fraction rich in C5+ gasoline hydrocarbons, and an aqueous phase. The fraction rich in C5+ gasoline hydrocarbon afterwards is supplied to a second separating means in which the aromatics are removed from the mixture. The remaining residual stream largely free of aromatics is at least partly recirculated into the reactor as recycling stream. This has the advantage that the olefin fraction for the most part can be converted to propylene, whereby the yield of propylene as a whole is increased.
WO 2008/039552 A1 teaches a process in which MTP® and cracking processes are connected in series. For this purpose, the oxygenates are converted into olefins in a reactor according to the MTP® process. From the product stream thus obtained, ethylene and propylene are separated. In this separation, remaining oxygenates and water also are removed from the stream, whereby a pure C4+ fraction is obtained, which is transferred into an olefin cracking reactor. By cracking the olefins, further ethylene and propylene can then be obtained.
US 2004/008 7824 A1 describes a combination of the two processes for converting oxygenates and for cracking olefins proceeding from the product mixture from a Fischer-Tropsch synthesis. For this purpose, the product stream which contains both oxygenates and C6+ olefins is brought in contact with an acid, olefin-cracking catalyst. Both the oxygenates and the high-molecular olefins then are converted to light olefins such as propylene, butene and pentene. The conversion is effected at temperatures between 260 and 454° C. and a pressure below 69 bar. Due to these reaction conditions, however, there is still obtained a considerable C4+ olefin fraction, whereas C4/C5 olefins are not significantly converted to lighter olefins and the valuable product ethylene can hardly be obtained.
The combination of an MTP® plant with a cracker plant is described in DE 10 2007 045 238 A1, wherein MTP® reactor and cracker are connected in parallel. The respective intermediate product streams of the steam cracker and the reactor are at least partly joined. This has the advantage that the succeeding separating means can be utilized jointly. Parts of these streams also are recirculated to the steam cracker and/or the MTP® reactor, whereby in particular in the MTP® reactor the longer-chain alkenes are cracked to lighter olefins, in particular ethylene and propylene.
With an interconnection according to DE 10 2007 045 238 A1, it is possible to produce 50,899 t.p.a. of ethylene and 440,331 t.p.a. of propylene more than in a pure cracker plant from 1,660,000 t.p.a of methanol in the combination of MTP® and cracker plants, and hence the ratio of C2 to C3 can effectively be shifted in favor of propylene from 43.06 to 1.86. However, the selectivity of the sum of the valuable products propylene and ethylene based on methanol only is about 0.68 and hence distinctly <1, from which it follows that the selectivity is only slightly larger than in a self-sufficient MTP® plant. This means that the increased yield of propylene chiefly is to be associated to the pure parallel connection of MTP® and cracker plant, and a substantial reduction of the olefins with higher C numbers present in the MTP®/cracker complex could not be effected.
From DE 10 2007 045 238 A1 it is also known that due to the presence of longer-chain olefins and as a result the occurrence of endothermal reactions, the temperature in the reactor can be lowered. However, it is not disclosed in the prior art in what ratio the streams of oxygenates and C4+ olefins must be, in order to keep the temperature in the reactor at an almost constant level. A rather uniform temperature profile, however, is important because the cracking of olefins so far is limited by the fact that the endothermal reaction makes the temperature in the reactor decrease, so that finally there will not be sufficient energy to overcome the activation energy, and therefore the reaction does not proceed completely. At the same time, the classical MTP® process is highly exothermal, so that despite an expensive cooling construction the rising temperature in the reactor reduces the selectivity with regard to propylene.
Therefore, it is the object of the invention to provide a process in which a maximization of the yield of propylene and ethylene can be achieved by a rather homogeneous temperature profile.